When are stateless class functors useful in place of a c style function?

Question

I\'ve found some good examples of functors on SO like this one, and all the convincing examples seem to use state in the class that defines operator().

Answer 1

Another reason is that sometimes one comparison function is not enough. Let's say we have a vector of pointers:

struct X { string name; };
vector> v;

Now if we want to sort the vector by name, we have to define our own predicate for the sort function:

struct Cmp1
{
    bool operator()(const shared_ptr& left, const shared_ptr& right) const
    { return left->name < right->name; }
};

That's cool, but what do we do when we need to find the objects with specific name? To work with equal_range, the predicate needs to have two different comparison functions:

struct Cmp2
{
    bool operator()(const shared_ptr& left, const string& right) const
    { return left->name < right; }

    bool operator()(const string& left, const shared_ptr& right) const
    { return left < right->name; }
};

This allows us to call equal_range with a string name object:

equal_range(v.begin(), v.end(), name, Cmp2())